Page 218 - Geochemistry of Oil Field Waters

SEDIMENTARY ROCKS 205

salts; or if they did, these salts were later removed by leaching so that their
occurrence is relatively rare.
Holser (1963) analyzed some brine inclusions in halite from Permian age
evaporites. He found that the Br/Cl and Mg/Cl ratios in many of the brine
inclusions are similar to those found in the late stages of halite deposition.
He concluded that some of the inclusions were connate bitterns with few
diagenetic changes, and that the Br/Mg ratio of sea water has remained
relatively constant since Permian time. Some diagenetic changes were evident
in a few of the inclusions in which a large ratio of Ca/C1 and a low ratio of
SO4 /Cl compared to sea water were found.
Sediments commonly associated with evaporites are red beds, quartzose
sandstones, subgraywacke sandstones, carbonate rocks, and marine shales
(Krumbein, 1951). Normal marine evaporite successions are found in inter-
cratonic basins such as the Michigan and Williston Basins. Euxinic black
shales sometimes are associated with evaporites. Low redox potentials have
been found in modern evaporite (Morris and Dickey, 1957; Quaide, 1958).
Examples of modern depositional evaporites are the Karaboghaz Gulf on the
eastern side of the Caspian Sea, the Great Bitter Lake of Suez, the Rann of
Cutch in northwest India (Grabau, 1920), and the Persian Gulf sabkhas
(Evans et al., 1963; Butler, 1969).

Deposition of organic matter

The organic matter can be biogenic (produced by living systems) or
abiogenic (not produced by living systems). The source of biogenic matter
can be both terrestrial and marine; for example, considerable plant and
animal debris is collected from the land by streams and rivers and carried to
the sea, while in the sea large quantities of plant and animal matter live and
die.
The organic matter that is deposited with sediment usually decomposes if
the conditions are right; however, if the environment is reducing some of it
may be preserved. The preserved organic matter is transformed into other
organic compounds (Kvenvolden, 1964). During sediment diagenesis the
organic matter is transformqd to insoluble organic matter (kerogen) and
soluble petroleum hydrocarbon (Hunt and Jamieson, 1958). Chemical,
bacterial, and catalytic reactions are involved in these conversions. Tempera-
ture and pressure affect the reaction rates. Some of the chemical reactions
are as follows:

(1) Oxidation : C2Hm + 02 -+ nC02 + '/znH,O
(2) Reduction: R'OH + H2 + R'H + H20
(3) Elimination: R'COOH + R'H + co2
(4) Polymerization: (small units) + big molecule
(5) Cracking: c-c-c-c-c-c~oc-c-c- + c-c